The present invention relates to methods of making semiconductor devices and in particular to methods of providing Schottky contacts to compound semiconductor layers utilized in semiconductor devices.
The fabrication and operation of diodes (junction and Schottky) and basic transistor devices are well known. New technologies have developed needs for higher speed and power transistors capable of withstanding extreme operating conditions such as high temperatures, current, and radiation. Silicon carbide devices have the potential to fulfill these needs but have yet to achieve commercial success. One obstacle to using silicon carbide in electronic devices is the difficulty in providing reliable and durable electrical contacts to the device.
Whenever a metal and a semiconductor are in intimate contact, there exists a potential barrier between the two that prevents most charge carriers (electrons or holes) from passing from one to the other. Only a small number of carriers have enough energy to get over the barrier and cross to the other material. When a bias is applied to the junction, it can have one of two effects: it can make the barrier appear lower from the semiconductor side, or it can make it appear higher. The bias does not change the barrier height from the metal side. The result of this is a Schottky Barrier (rectifying contact), where the junction conducts for one bias polarity, but not the other. Almost all metal-semiconductor junctions will exhibit some of this rectifying behavior.
Schottky barrier diodes (SBD's) in Si are only practical to around 70V-100V, limited by excess leakage under reverse bias. Using SiC allows use of SBD's at much higher voltages, up to 1200V. Diodes are used as high-voltage rectifiers in many power switching applications. Whenever current is switched to an inductive load such as an
electric motor, high-voltage transients are induced on the lines. To suppress these transients, diodes are placed across each switching transistor to clamp the voltage excursions. PN junction diodes can be used for this application, but they store minority carriers when forward biased, and extraction of these carriers allows a large transient reverse current during switching. Schottky barrier diodes are rectifying metal-semiconductor junctions, and their forward current consists of majority carriers injected from the semiconductor into the metal. Consequently, SBD's do not store minority carriers when forward biased, and the reverse current transient is negligible. This means the SBD can be turned off faster than a PN diode, and dissipates negligible power during switching. SiC Schottky barrier diodes are especially attractive because the breakdown field of SiC is about eight times higher than in silicon, enabling much higher voltage operation compared to Si. SiC Schottky barrier diodes can be used in place of Si PN diodes, enabling faster switching speeds and greater efficiency. In addition, because of the wide bandgap, SiC SBD's should be capable of much higher temperature operation than silicon devices.
However, conventional SiC Schottky barrier diodes experience problems. Many conventional SiC Schottky diodes use nickel (Ni) as a metal for the Schottky contact.
The Ni is typically annealed on the backside to provide an ohmic contact and left unannealed on the front side to provide the Schottky contact. Ni is reactive with SiC and even at low temperatures the reaction proceeds until the Ni Schottky contact becomes ohmic. At high current densities, the Ni-SiC diodes become hot and, over time, tend to anneal the Schottky contact. Given temperature variations and time and the current carried by the Ni-SiC diodes, it is virtually inevitable that they will degrade and become resistors. This can have catastrophic consequences for the electronic device or system that is relying upon the Ni-SiC Schottky diode to conduct or block current.
The ternary phase diagram was calculated for the system composed of rhenium, silicon and carbon. The ternary diagram is calculated assuming constant temperature, pressure, and the neglect of ternary compounds of Re, Si, and C. To calculate the ternary phase diagram, the Gibbs Phase rule is employed, which demands that at constant temperature and pressure only three phases are allowed simultaneously. This is discussed in References 1 and 2. In a practical sense this means that tie lines—lines which connect compounds and elements that are thermodynamically stable with each other and do not react—are not allowed to cross in the diagram, because at that point 4 phases would be allowed to exist, which is forbidden by Gibbs' Phase Rule. These tie lines are calculated by considering the Gibbs Free Energy of all of the possible reactions of the constituent compounds and elements of the system (in this case, Re, Si, C, and the Re suicides). Based on the Gibbs Free Energy, if none of the reactions proceed for any given pair of reactants, the pair is connected by a tie line. This procedure is repeated until all possible pairs of reactants have been considered. The resulting ternary phase diagram is shown in FIG. 6. Thermodynamic data for the calculation is taken from Reference 3.
The ternary phase diagram in FIG. 6 makes quite clear that Re is thermodynamically stable with respect to SiC over the temperature range from 300 to 1100 K. This means that a film of Re will not react with SiC to form Re silicides over this temperature range. No Re carbides are stable in the temperature range of 300 to 1100 K. An example of the stability of Re on SiC has been discussed in Reference 4. Reference 4 shows Re stable on beta-SiC at 1100 ° C.
As to first order the electrical properties of a system are governed by physical properties, the fact that the Re will not react with SiC means that because the Re contact is a Schottky barrier, it will remain a Schottky barrier. When, but not until, the Re-SiC system temperature increases to over 1100K, the Re reacts to form a Re-silicide, and the electrical properties may change. The Re/SiC ternary phase diagram makes quite clear the long lifetime and durability of the Re-SiC contact, useful for devices from diodes to MESFETs.